PM shape changes in preparation for fusion. (A–C) Virtual z-slices through representative tomograms of mating cells showing sPM versus wPM (the predominant situation; A), sPM in both (B), and wPM in both cells (C). Segmentation models of the PM outer leaflet and schematic drawings are shown at the right. (D) Percentage of cells with wPM along fusion stages (n = 26 far CW contact cells; n = 100 close CW contact; n = 22 PM contact and n = 86 after fusion). (E–G) Virtual slices through representative tomograms of mating cells showing symmetric (convex versus convex; E) and asymmetric PM curvatures (convex versus flat [F] and convex versus concave [G]). Segmentation models of the PM outer leaflet and schematic drawing are shown at the right. (H) Percentage of asymmetric mating pairs along fusion stages (n = 13 far CW contact pairs; n = 50 close CW contact; n = 11 PM contact and n = 43 after fusion). (I) Percentage of cells with sPM or wPM in prefusion pairs showing a convex–concave configuration, showing that low PM tension is associated with the concave state. n = 20 pairs. Scale bars, 100 nm.